Thermal printing ink ribbons have long been used as a means for printing on various articles information such as bar code images. These thermal printing ink ribbons comprise an ink layer consisting of a colorant and a binder material such as wax on a heat-resistant base, and may be fitted to a printer so that the ink layer is molten under heat of the thermal head and transferred onto package and paper or a label to give a desired thermal bar code image thereon.
In recent years, transferred images that are printed using such a thermal printing ink ribbon have been required to be extremely sharp in appearance. One method of obtaining sharper transferred images includes the addition of fine particles of a heat setting resin such as silicon resin or the like to the ink layer of a thermal transfer ink ribbon whose binder mainly comprises wax. This improves the ability of the ink layer to be separated sharply from the base and results in a sharper image.
Another method includes using a thermally fusible resin such as a thermoplastic resin, instead of wax, as a main constituent of the binder. While the use of a thermoplastic resin is effective to improve heat resistance and wear resistance of transferred images, however, the ability of the ink layer to be sharply transferred from the base is reduced. This fails to produce an image of sufficient clarity. Resin binders and/or waxes of higher melting points can also provide a higher degree of scratch and smear resistance. However, higher print head energies are necessary to achieve the desired flow to promote transfer and adhesion to a receiving article.
There are some limitations on the applications for thermal transfer printing. For example, the properties of the thermal transfer formulations which permit transfer from the carrier to a receiving substrate can place limitations on the permanency of the printer matter. Printed matter from conventional processes can smear or smudge, especially when subjected to a subsequent sorting operation. Additionally, where the surface of a receiving substrate is subject to scratching the problem is compounded. This smearing can make character recognition such as optical character recognition or magnetic ink character recognition difficult and sometimes impossible. In extreme cases, smearing can make it difficult to read bar codes. Additionally, exposure of the image to various chemicals can be detrimental.
U.S. Pat. No. 6,025,017 discloses a UV or visible light curable coating formulation which uses monomers and oligomers for the purpose of reducing or eliminating solvents during the manufacturing process. This coating is cured during the manufacture of the ribbon, prior to any printing, to form a thermoplastic polymer that can be thermally transferred to a receiving substrate.
U.S. Pat. No. 6,040,040 discloses a radiation curable thermal printing ink which is selectively cured during ribbon manufacture, prior to printing. The ink is applied to a substrate in multiple or graded layers.
U.S. Pat. Nos. 5,919,557 and 5,952,098 relate to a thermal transfer medium having reactive components that cross-link when heated. Radiation-curable components are not disclosed.
Many attempts have been made to provide high integrity thermal transfer printing which is resistant to degradation due to chemical, heat and physical damage, some of which are described above. There is a continuing effort to provide alternative thermal transfer media which can form printed images with high resistance to these kinds of damage.